Howdy, y'all. Whenever I start my bike cold, I get one single gargantuan explosion out the exhaust pipe. Sounds like a mega caliber rifle shot. The bike starts roughly immediately afterwards, and runs fine as soon as I get the rpms up. Heavy smell of gas in the exhaust until the rpms go up and I open the choke.

If I lived in town, I'd've probably asked about this months ago, but my nearest neighbor is a quarter mile away. As it is, my 6:00 a.m. sonic booms are making my wife a bit angry and I'm sort of worried about the upcoming winter. Will it get worse? Will I eventually blow up the muffler? Is a valve not closing completely?

Anyone else have a similar time of it? What helped?

Thanks.
Irishman

92 GS500
26k miles
recent air filter cleaning to no effect
Valves have not been adjusted since I bought it at 13k miles

I think the last sentence of your post is a red flag........

Quote from: Big Rich on November 08, 2012, 03:10:18 AM
I think the last sentence of your post is a red flag........

X10.  Valves should be checked and adjusted as necessary ever 4000 or so miles per the Wiki:  http://wiki.gstwins.com/index.php?n=Maintenance.MaintenanceSchedule  Look for "Tappet Clearance".

13K is a Looong time.

Figured that might be the likely place to start. Thanks.

if you open the throttle when you start it, you introduce a an inordinate amount of air to the system which usually ends up as a non-combustion, goes to the tail pipe, and when the next fuel rich mixture ignites, all that fuel air mix in the tailpipe...


well you know.... 

if you aren't opening the throttle when you try to start her up, then valves are likely the problem.

Your valve clearances really should be checked after 13k miles of use but if you have cold compression that is not likely your problem and cold compression can be checked quickly.  Most likely it's a problem of ignition missfiring, I've heard that big blast more than a few times on different bikes including my GSs riding year around here in NE Ohio.  A cylinder is not firing when you start cranking and when it finally does fire it ignites the accumulated mixture in the exhast and bang.  On a very cold day it can simply be the engine with cold heavy oil in it draws down the voltage so low while cranking that there isn't enough left to bridge the plug gaps.  If it's doing that regularly you could try a jumper on the battery when starting and see if that helps.  One of the first things I'd check on a GS is if the plug wires are tight in the coils.  The wires should be tightly bonded into the coils and you shouldn't be able to turn them.  On a GS that old just take the wire off the plug and pull hard on it to see if you can pull it out of the coil for starters.

Thanks, Jack. Will check that tonight or tomorrow.

All GS500 models (and I suspect most other motorcycles) have really dumb ignition systems - old and new.

There is the timing rotor to indicate where the crank is. When a piston is at the top, it fires the spark.

Unfortunately it knows not whether the piston is on the compression cycle or the exhaust cycle. Remember it is 2 piston cycles because we have a 4 stroke engine.

because there is no way to know which cycle the piston is on, you get a spark on both compression and exhaust. If compression firing was poor or missed, you could get a firing in the exhaust cycle. That would be exhaust popping. I suppose depending on timing and valve openings and closings, you could get a backfire through the intake.

Somewhere someone mentioned the intake and exhaust are momentarily open at the same time at the top of the exhaust cycle as intake starts. I have not examined that but timing off a little I suppose odd pops and backfires can happen.

Actually the intake valve opens 34* before TDC and the exhaust valve closes 37* after TDC so both are open for 71* when the spark fires at TDC during the scavanger cycle.  So if the plug is firing it burns the left over fuel/air mix as it is exausted each cycle and does not contribute to the big bang resulting when the plug is not firing at the top of the power stroke and the full unburned fuel/air charges accumulate on the exhaust system and the plug then fires and the exhausting fire blows the whole mess and even the exhaust system too on some old vehicles.   If you've heard it you know it for sure!

Checked the plug wires last night and they seemed okay. Pulled the spark plugs and they were pretty worn--edges all rounded and coated in ash. I cleaned and re-gapped them and the bike started and no backfire this morning, though I rode it fairly late last night so it might've been warmer than normal. New plugs are on the way, and I'll let you know how it goes.

Thanks.

Irishman

So am I reading this right that when starting the bike, one should not twist the throttle before or during the startup procedure?